The research on substitute energy to fossil fuel has gaining more importance by the day. Hydrogen is a substitute energy which is recyclable, safe and clean. From the considerations of safety, convenience, and energy density, hydrogen storage is preferably carried out by adsorption of hydrogen on a metal to form a non-explosive metal hydride solid. The Department of Energy (DOE) of the U.S.A. has published a plan that researches on the storage of hydrogen in the future are concentrated on low temperature hydrogen storage alloys and hydrogen storage carbonaceous material. To be used as a power source for an automobile using a fuel cell, the energy density of hydrogen storage for a hydrogen storage system needs to be 6.5 wt % or 60 kg/m3 and a travel range for the hydrogen storage system is of 350 miles.
A hydrogen storage alloy is formed of an element having a strong affinity with hydrogen, and capable of existing in a compound state after being combined with the hydrogen molecule. The existing hydrogen storage alloys mainly include: rare earth series, titanium series, zirconium series, and magnesium series; or can be classified into the following types according to the atomic ratio of the major constituting elements: AB, A2B, AB2, AB5, AxB, ABx, wherein A is a hydrogen binding element, B is an element with catalytic function, regulation function, or special function.
To achieve a hydrogen storage target of 6 wt %, only the magnesium series alloys are available at the present, for example those disclosed in U.S. Pat. No. 6,193,929 (which is incorporated herein by reference). However, such an alloy is not practical due to the restriction that the alloy releases hydrogen at a temperature exceeding 300° C. So far, low temperature hydrogen storage alloys developed can only store about 2 wt % of hydrogen, and most of them are titanium series or lanthanum-nickel series alloys.